Intranasal and Inhaled Vaccines

Many respiratory infections have epidemic and pandemic potential, as evidenced by influenza and the recent COVID-19 pandemic. Vaccination is an established and effective strategy to contain the spread of communicable diseases. Most vaccines are injectables that induce potent systemic immunity. However, they are less efficient in eliciting immune responses at the respiratory mucosa, the site where many respiratory pathogens invade and replicate. This therapeutic inadequacy may be addressed by intranasal and inhaled vaccines which can trigger mucosal immunity. Only limited intranasal and inhaled vaccines are clinically available, including a live-attenuated influenza vaccine and two recently marketed viral vector vaccines targeting SARS-CoV-2. Nevertheless, the immunological advantages of mucosal vaccines should not be undermined, which include the activation of tissue-specific mucosal immune response and common mucosal immune system, while they also induce systemic immunity as injectable vaccines do. In this chapter, the mechanisms of intranasal and inhaled vaccines are introduced and their advantages over injectable vaccines are discussed. Other social and economic benefits of intranasal and inhaled vaccines are also presented, followed by an overview of associated challenges and limitations that must be overcome and addressed before their therapeutic potential can be better utilised.